Capacity modulation of refrigerant vapor compression system

ABSTRACT

A carbon dioxide refrigerant vapor compression system and a method of operating that system are provided. The refrigerant vapor compression system includes a compression device, a refrigerant heat rejection heat exchanger and a refrigerant heat absorption heat exchanger disposed in a primary refrigerant circuit, and a compression device unload circuit in parallel refrigerant flow relationship with the primary refrigerant circuit. At full load, the primary refrigerant circuit is open, but the unload circuit is closed. To modulate capacity, a controller alternates operation between a first cycle for a first period of time wherein the primary circuit is open and the unload circuit is closed and a second cycle for a second period of time wherein the primary circuit is closed and the unload circuit is open.

FIELD OF THE INVENTION

This invention relates generally to refrigerant vapor compressionsystems and, more particularly, to efficient capacity modulation ofrefrigerant vapor compression systems, including transport refrigerationrefrigerant vapor compression systems using carbon dioxide refrigerantand operating in a transcritical cycle.

BACKGROUND OF THE INVENTION

Refrigerant vapor compression systems are well known in the art andcommonly used in transport refrigeration systems for refrigerating airsupplied to a temperature controlled cargo space of a truck, trailer,container or the like for transporting perishable items. Refrigerantvapor compression systems are also commonly used in commercialrefrigeration systems associated with supermarkets, convenience stores,restaurants, and other commercial establishments for refrigerating airsupplied to a cold room or a refrigerated display merchandiser forstoring perishable food items. Refrigerant vapor compression systems arealso commonly used for conditioning air to be supplied to a climatecontrolled comfort zone within a residence, office building, hospital,school, restaurant or other facility. Typically, such refrigerant vaporcompression systems include a compressor, an air-cooled condenser, anevaporator and an expansion device, commonly a thermostatic orelectronic expansion valve, disposed upstream, with respect torefrigerant flow, of the evaporator and downstream of the condenser.These basic refrigerant system components are interconnected byrefrigerant lines in a closed refrigerant circuit, arranged in accordwith known refrigerant vapor compression cycles.

Traditionally, most of these refrigerant vapor compression systemsoperate at subcritical refrigerant pressures. Refrigerant vaporcompression systems operating in the subcritical range are commonlycharged with conventional fluorocarbon refrigerants such as, but notlimited to, hydrochlorofluorocarbons (HCFCs), such as R22, and morecommonly hydrofluorocarbons (HFCs), such as R134a, R410A and R407C. Intoday's market, greater interest is being shown in “natural”refrigerants, such as carbon dioxide, for use in air conditioningapplications, commercial refrigeration applications, and transportrefrigeration applications instead of HFC refrigerants. However, becausecarbon dioxide has a low critical temperature, most refrigerant vaporcompression systems charged with carbon dioxide as the refrigerant aredesigned for operation in the transcritical pressure regime. Forexample, transport refrigerant vapor compression systems having an aircooled refrigerant heat rejection heat exchanger operating inenvironments having ambient air temperatures in excess of the criticaltemperature point of carbon dioxide, 31.1° C. (88° F.), must alsooperate at a compressor discharge pressure in excess of the criticalpoint pressure for carbon dioxide, 7.38 MPa (1070 psia) and thereforewill operate in a transcritical cycle. In refrigerant vapor compressionsystems operating in a transcritical cycle, the refrigerant heatrejection heat exchanger operates as a gas cooler rather than acondenser and operates at a refrigerant temperature and pressure inexcess of the refrigerant's critical point, while the evaporatoroperates at a refrigerant temperature and pressure in the subcriticalrange.

In transport refrigerant applications, the refrigerant vapor compressionsystem must be capable of operating in a pull-down mode and in a setpoint control mode. The refrigerant vapor compression system operates inthe pull-down mode when the cargo space is loaded with perishable itemshaving a temperature significantly in excess of the desired storagetemperature for transport of those items. For example, it is customaryto load fruit and vegetables directly from the picking field into thecargo storage space of a truck, a trailer or an intermodal transportcontainer. Thus, it is desirable to rapidly reduce the temperature ofthe product within the cargo storage space from an ambient temperatureat which the product was loaded to a desired storage temperature fortransport, typically between about 1° C. to about 5° C. (about 34° F. toabout 40° F.) for refrigerated food items and under 0° C. (32° F.) forfrozen food items. Accordingly, the refrigerant vapor compression systemmust be designed with sufficient capacity at full load operation tosufficiently cool the air circulating from the cargo storage spacethrough the evaporator of the refrigerant vapor compression system torapidly draw down the temperature of the product within the cargostorage space.

However, once the cargo storage space has been cooled down to thedesired storage temperature for transport of the particular cargo beingtransported, the refrigeration vapor compression system operates in aset point control mode. In this operating mode, the refrigerant vaporcompression system must maintain the temperature within the cargostorage space within a relatively narrow plus/minus range of a set pointtemperature equal to the desired transport temperature for theparticular product stored therein. To avoid over cooling of the product,the refrigerant vapor compression system must be operated at a reducedrefrigeration capacity substantially less than the full load refrigerantcapacity of the system to avoid over cooling of the air circulating fromthe cargo storage space.

SUMMARY OF THE INVENTION

In an aspect of the invention, a refrigerant vapor compression systemincludes a refrigerant compression device, a refrigerant heat rejectionheat exchanger, an expansion device and a refrigerant heat absorptionheat exchanger disposed in serial refrigerant flow communication in aprimary refrigerant circuit, and an unload circuit operativelyassociated with said compression device. A first flow control device isdisposed in the primary refrigerant circuit downstream with respect torefrigerant flow of the discharge outlet of the compression device andupstream with respect to refrigerant flow of the refrigerant heatrejection heat exchanger. The unload circuit includes an unloadrefrigerant line having an inlet in refrigerant flow communication withthe primary refrigerant circuit at a first location downstream withrespect to refrigerant flow of the discharge outlet of the compressiondevice and upstream with respect to refrigerant flow of the first flowcontrol device, and having an outlet in refrigerant flow communicationwith the primary refrigerant circuit at a second location downstreamwith respect to refrigerant flow of said refrigerant heat absorptionheat exchanger and upstream with respect to refrigerant flow of thesuction inlet to the compression device, and a unload circuit flowcontrol device disposed in the unload refrigerant line. The refrigerantvapor compression system further includes a controller operativelyassociated with the first flow control device and the unload circuitflow control device. The controller operates to switch the refrigerantvapor compression system between a first operating mode wherein thecompression device operates in a loaded cycle and a second operatingmode wherein the compression device operates in an unloaded cycle.

The controller, in a first operating mode, positions the unload circuitflow control device in a closed position and positions the first flowcontrol device in an open position to operate the refrigerant vaporcompression system in a loaded cycle. In a second operating mode, thecontroller positions the unload circuit flow control device in an openposition and positions said first flow control device in a closedposition to operate the refrigerant vapor compression system in unloadedcycle. In the first operating mode, the controller may also modulate theexpansion device. In the second operating mode, the controller may alsoposition the expansion device in a closed position.

In an embodiment, the refrigerant vapor compression system furtherincludes a second flow control device disposed in the primaryrefrigerant circuit downstream with respect to refrigerant flow of therefrigerant heat absorption heat exchanger and upstream with respect torefrigerant flow of the suction inlet of the compression device. In thisembodiment, the outlet of the unloaded refrigerant line is inrefrigerant flow communication with the primary refrigerant circuit at asecond location downstream with respect to refrigerant flow of thesecond flow control device and upstream with respect to refrigerant flowof the suction inlet to the compression device. In this embodiment, inthe first operating mode, the controller positions the unload circuitflow control device in a closed position and positions each of the firstflow control device and the second flow control device in a openposition to operate the refrigerant vapor compression system in a loadedcycle. In the second operating mode, the controller positions the unloadcircuit flow control device in a open position and positions each of thefirst flow control device and the second flow control device in a closedposition to operate the refrigerant vapor compression system in anunloaded cycle.

In an aspect of the invention, a method is provided for modulating thecapacity of a refrigerant vapor compression system including arefrigerant compression device, a refrigerant heat rejection heatexchanger, an expansion device, and a refrigerant heat absorption heatexchanger disposed in series flow arrangement in a primary refrigerantcircuit, including the steps of: operating the compression device in aloaded cycle for a first period of time; operating the compressiondevice in an unloaded cycle for a second period of time; and repeatedlyalternating operation of the compression device between operation in theloaded cycle and the unloaded cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of these and other objects of the invention,reference will be made to the following detailed description of theinvention which is to be read in connection with the accompanyingdrawing, where:

FIG. 1 is a schematic diagram illustrating a first exemplary embodimentof a refrigerant vapor compression system in accord with the invention;

FIG. 2 is a schematic diagram illustrating a second exemplary embodimentof a refrigerant vapor compression system in accord with the invention;and

FIG. 3 is a schematic diagram illustrating a third exemplary embodimentof a refrigerant vapor compression system in accord with the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIGS. 1-3, the refrigerant vapor compression system 10will be described herein in connection with the refrigeration of atemperature controlled cargo space 200 of a refrigerated container,trailer or truck for transporting perishable items. It is to beunderstood, however, that the refrigerant vapor compression systemdescribed herein could also be used in connection with refrigerating airfor supply to a refrigerated display merchandiser or cold roomassociated with supermarket, convenience store, restaurant, or othercommercial establishment or for conditioning air to be supplied to aclimate controlled comfort zone within a residence, office building,hospital, school, restaurant or other facility. The refrigerant vaporcompression system 10 includes a compression device 20 driven by a motor30 operatively associated therewith, a refrigerant heat rejection heatexchanger 40, a refrigerant heat absorption heat exchanger 50, connectedin a closed loop refrigerant circuit in series refrigerant flowarrangement by various refrigerant lines 2, 4 and 6. Additionally, anexpansion device 55, operatively associated with the evaporator 50, isdisposed in refrigerant line 4 downstream with respect to refrigerantflow of the refrigerant heat rejection heat exchanger 40 and upstreamwith respect to refrigerant flow of the refrigerant heat absorption heatexchanger 50. In the embodiment of the refrigerant vapor compressionsystem 10 depicted in FIGS. 1 and 3, the expansion device 55 comprisesan electronic expansion valve. However, in the embodiment depicted inFIG. 2, the expansion device 55 comprises a thermostatic expansion valve57, or a fixed orifice device, such as a capillary tube, disposed inseries refrigerant flow with an open/closed flow control device 53, suchas a two-position solenoid valve.

When the refrigerant vapor compression system 10 is operating in asubcritical cycle, such as when charged with a conventionalhydrochlorofluorocarbon refrigerant (HCFC), such as R22, orhydrofluorocarbon refrigerant (HFC), such as R134a, R410A and R407C, orcarbon dioxide refrigerant when operating at a compressor dischargepressure below the critical point pressure of carbon dioxide of 7.38 MPa(1070 psia), the refrigerant heat rejection heat exchanger 40 operatesin the subcritical pressure range and functions as a refrigerant vaporcondenser. However, when the refrigerant vapor compression system 10 isoperating in a transcritical cycle, such as when charged with carbondioxide refrigerant and operating at a compressor discharge pressure inexcess of the critical pressure point of carbon dioxide, the refrigerantheat rejection heat exchanger 40 operates at supercritical pressure andfunctions as a refrigerant vapor cooler, but does not operate tocondense the carbon dioxide refrigerant vapor. The tube bank 42 of theheat rejection heat exchanger 40 may comprise, for example, a finnedtube heat exchanger, such as for example a plate fin and round tube heatexchanger coil, or a fin and multi-channel tube heat exchanger, such asfor example fin and minichannel or microchannel flat tube heatexchanger. In traversing the refrigerant heat rejection heat exchanger40, the refrigerant passes through the heat exchanger tubes of the tubebank 42 in heat exchange relationship a secondary fluid, typicallyambient air, generally outdoor air, being drawn through the tube bank 42by an air mover 45, such as one or more fans, operatively associatedwith the tube bank 42 of the heat rejection heat exchanger 40.

Whether the refrigerant vapor compression system 10 is operating in asubcritical cycle or a transcritical cycle, the refrigerant heatabsorption heat exchanger 50, being located in the refrigerant circuitdownstream with respect to refrigerant flow of the expansion device 55,always operates at a subcritical pressure and functions as a refrigerantliquid evaporator. In traversing the heat absorption heat exchanger 50,the refrigerant passes through the heat exchanger tubes of the tube bank52 in heat exchange relationship with air to be conditioned, typicallyair drawn from and to be returned to a climate-controlled environment,being drawn through the tube bank 52 by an air mover 55, such as one ormore fans, operatively associated with the tube bank 52 of the heatabsorption heat exchanger 50, whereby the air is cooled and therefrigerant is heated and evaporated. The tube bank 52 of therefrigerant heat absorption heat exchanger 50 may comprise, for example,a finned tube heat exchanger, such as for example a plate fin and roundtube heat exchanger coil, or a fin and multi-channel tube heatexchanger, such as for example a fin and minichannel or microchannelflat tube heat exchanger.

The compression device 20 functions to compress and circulaterefrigerant through the refrigerant circuit as will be discussed infurther detail hereinafter. The compression device 20 may be a single,single-stage compressor as depicted in FIGS. 1 and 2, such as forexample a scroll compressor, a reciprocating compressor, a rotarycompressor, a screw compressor, a centrifugal compressor. However, it isto be understood that the compression device 20 may also be amultiple-stage compression device having at least a lower pressurecompression stage and a higher pressure compression stage withrefrigerant flow passing from the lower pressure compression stage tothe higher pressure compression stage, such as depicted in FIG. 3. Insuch an embodiment, the multiple-stage compression device may comprise asingle, multi-stage compressor, such as, for example, a scrollcompressor, or a screw compressor having staged compression pockets, ora reciprocating compressor having at least a first bank of cylinders anda second bank of cylinders, or a pair of single-stage compressorsconnected in series refrigerant flow relationship with the dischargeoutlet of the upstream compressor connected in serial refrigerant flowcommunication with the suction inlet of the downstream compressor

The drive motor 30 operatively associated with the compression mechanismof the compression device 20 may be a fixed-speed motor operating onpower from a fixed frequency power source. The compression device 20receives refrigerant vapor at suction pressure from the evaporator 50through the refrigerant line 2 connected in refrigerant flowcommunication to the suction inlet of the compression device 20 anddischarges refrigerant vapor at discharge pressure to the refrigerantheat rejection heat exchanger 40 through refrigerant line 2 connected inrefrigerant flow communication with the discharge outlet of thecompression device 20. A flow control device 65 is interdisposed inrefrigerant 6 at a location downstream with respect to refrigerant flowof the evaporator 50 and upstream with respect to refrigerant flow ofthe suction inlet of the compression device 20. Additionally, a flowcontrol device 75 is interdisposed in refrigerant 2 at a locationdownstream with respect to refrigerant flow of the discharge outlet ofthe compression device 20 and upstream with respect to refrigerant flowof the refrigerant heat rejection heat exchanger 40. Each of the flowcontrol devices 65, 75 may be selectively positioned in at least a fullyopen position wherein refrigerant may flow through the flow controldevice and in a fully closed position wherein refrigerant can not flowthrough the flow control device. In an embodiment, each of the flowcontrol devices 65, 75 comprise a two-position solenoid valve having anopen position and a closed position.

Referring now to FIG. 3, in the exemplary embodiment of the refrigerantvapor compression system 10 depicted therein, the primary refrigerantcircuit includes an economizer circuit in operative associationtherewith. The economizer circuit includes an economizer refrigerantline 14, an economizer expansion device 73, an economizer heat exchanger70 and an economizer flow control valve 95. The economizer refrigerantline 14 establishes refrigerant flow communication between therefrigerant line 4 of the primary refrigerant circuit and anintermediate pressure stage of the compression process. The economizerheat exchanger 70 may comprise a refrigerant-to-refrigerant heatexchanger having a first refrigerant flow pass 72 and second refrigerantflow pass 74. The first refrigerant flow pass 72 is interdisposed in therefrigerant line 4 of the primary refrigerant circuit downstream withrespect to refrigerant flow of the refrigerant outlet of the refrigerantheat rejection heat exchanger 40 and upstream with respect torefrigerant flow of the expansion device 55. The second refrigerant flowpass 74 is interdisposed in economizer refrigerant line 14. Theeconomizer expansion device 73, which may be an electronic expansionvalve, a thermostatic expansion valve, or a fixed orifice flow meteringdevice, is disposed in economizer refrigerant line 14 upstream withrespect to refrigerant flow therethrough of the second pass 74 of therefrigerant-to-refrigerant heat exchanger 70. The economizer flowcontrol device 95, which may be a two position, open/closed solenoidvalve, is interdisposed in the economizer refrigerant line 14 downstreamof the second pass 74 of the economizer heat exchanger 70. In operationof the primary refrigerant circuit, the controller 100 may selectivelyopen or close the economizer flow control device 95 to bring theeconomizer circuit on line or take it off line as in conventionalpractice to switch between economized and non-economized refrigerationcycles. If the expansion device 73 is an electronic expansion valve, theeconomizer flow control valve 95 may be omitted and the controller 100may open and close the electronic expansion valve to bring theeconomizer circuit on line or take it off line.

The refrigerant vapor compression system 10 further includes acompressor unload circuit comprising an unloader refrigerant line 8 thatinterconnects refrigerant line 2 of the refrigerant circuit withrefrigerant line 6 of the refrigerant circuit and an unloader valve 85disposed in the unloader refrigerant line 8 that is operative to controlthe flow of refrigerant through unloader the refrigerant line 8 of thecompressor unload circuit. In an embodiment, the unloader valve 85comprises a two-position solenoid valve having an open position and aclosed position. The unloader refrigerant line 8 taps into refrigerantline 2 at a location between the compression device 20 and the flowcontrol valve 75, that is, downstream with respect to refrigerant flowof the discharge outlet of the compression device 20 and upstream withrespect to refrigerant flow of the flow control valve 75 and taps intorefrigerant line 6 a location between the flow control flow valve 65 andthe compression device 20, that is, downstream with respect torefrigerant flow of the flow control valve 65 and upstream with respectto refrigerant flow of the suction inlet of the compression device 20.Thus, when the unloader control valve 85 is positioned in its openposition, refrigerant vapor may flow from the discharge outlet of thecompression device 20 through the unloader refrigerant line 8 directlyback to the suction inlet of the compression device 20.

The refrigerant vapor compression system 10 also includes a controller100 operatively associated with each of the respective flow controldevices 65, 75 and 85 interdisposed in refrigerant lines 6, 2 and 8,respectively, for selectively positioning each of the respective flowcontrol devices in an open or a closed position. The controller 100 alsomonitors the temperature of the ambient air passing into the refrigerantheat rejection heat exchanger 40 as the cooling medium via temperaturesensor 101, the temperature of the air and/or the product within thetemperature controlled cargo storage space 200 via temperature sensor201, and various system operating parameters by means of various sensorsoperatively associated with the controller 100 and disposed at selectedlocations throughout the system. For example, in the exemplaryembodiments depicted in FIGS. 1-3, a temperature sensor 103 and apressure sensor 104 may be provided to sense the refrigerant suctiontemperature and pressure, respectively, and a temperature sensor 105 anda pressure sensor 106 may be provided to sense refrigerant dischargetemperature and pressure, respectively. The pressure sensors may beconventional pressure sensors, such as for example, pressuretransducers, and the temperature sensors may be conventional temperaturesensors, such as for example, thermocouples or thermistors.

The controller 100 controls operation of the refrigerant vaporcompression system 10 and selective positioning of the flow controldevices 65, 75 and 85, and the economizer flow control valve 95, ifpresent. The controller 100 also controls operation of the compressordrive motor 30 driving the compression mechanism of the compressiondevice 20, as well as operation of the fans 44 and 54, through controlof the respective fan motors (not shown) operatively associatedtherewith. The controller 100 determines the desired mode of operationbased upon a comparison of the sensed temperature of the air and/orproduct within the cargo storage space 200 and a set point temperaturerepresentative of the desired storage temperature during transport forthe product stored therein. If the temperature of the product within thecargo storage space 200 exceeds the set point temperature by more than afew degrees, such as is the case upon initial start-up of therefrigerant vapor compression system 10 after loading product into thespace 200, the controller 100 operates the refrigerant vapor compressionsystem 10 at high capacity in a pull down mode. However, if thetemperature of the product within the cargo storage space 200 is withina preselected range of the set point temperature, the controller 100operates the refrigerant vapor compression system 10 at a reducedcapacity in a set point control mode.

To operate the refrigerant vapor compression system 10 in a pull downmode, the controller 100 closes the unloader valve 85 and opens both thesuction flow control valve 65 and the discharge flow control valve 75 sothat refrigerant circulates through refrigerant lines 2, 4 and 6 theprimary refrigerant circuit, but not through refrigerant line 8 of theunloader circuit. The controller 100 selectively opens the unloadercircuit control device 75, which comprises a fixed flow area valve, suchas for example a fixed orifice solenoid valve, in response to the sensedair temperature entering the evaporator from the temperature controlledspace 200 which is indicative of the temperature of the air or productwithin the temperature controlled space. However, it is to be understoodthat other system parameters may be used by the controller 100 indetermining when to open the unloader circuit control valve 85.

In the embodiments depicted in FIGS. 1 and 3, in the pull down mode, thecontroller 100 also modulates the flow of refrigerant to the evaporator50 by varying the flow area of the flow passage through the electronicexpansion valve 55, in response to the refrigerant suction temperatureor pressure, sensed by the sensors 103 and 104, respectively, on thesuction side of the compression device 20. If the suction pressure dropstoo low, it is an indication that the system is providing too muchcapacity. In the embodiment depicted in FIG. 3, in the pull down mode,the controller 100 may also selectively switch between operation in anon-economized refrigeration cycle and operation in an economizedrefrigerant cycle by selectively opening or closing the economizer flowcontrol valve 95.

The controller 100 can unload the compression device 20 as necessary tocontrol the refrigeration capacity of the refrigerant vapor compressionsystem 10. For example, when the sensed temperature of the air and/orproduct within the cargo storage space 200 has been reduced to within afew degrees of the set point temperature representative of the desiredtemperature for transporting the product stored therein, the controller100 switches operation from the pull down mode to the set point controlmode. To do so, the controller 100 opens the flow control device 85interdisposed in unloader refrigerant line 8 and simultaneously closesboth of the flow control valves 65 and 75 interdisposed in the primaryrefrigerant circuit, as well as the economizer flow control valve 95, ifpresent. With the unloader flow control device 85 open, the refrigerantvapor discharging from the compression device 20 passes through theunload circuit refrigerant line 8 to return directly to the suction sideof the compression device 20, thereby unloading the compression device20. This unloading of the compression device 20 through the unloadcircuit may also be implemented in response to a high compressordischarge refrigerant temperature or pressure.

To operate the refrigerant vapor compression system in the set pointcontrol mode, the controller 100 modulates the refrigeration capacity ofthe refrigerant vapor compression system 10 by selectively loading andunloading the compression device 20. The controller 10 does so byalternately opening and closing the unloader flow control device forshort periods of time, while simultaneously closing and opening the flowcontrol devices 65 and 75. Thus, the controller 100 operates the system10 in a load cycle for a first period of time by closing the unloaderflow control device 85 and, in synchronization therewith, opening theflow control devices 65 and 75, then operates the system 10 in an unloadcycle for a second period of time by opening the unloader flow controldevice 85 and, in synchronization therewith, closing the flow controldevices 65 and 75.

In an alternate embodiment of the refrigerant vapor compression system10, the flow control device 65 may be deleted. In this case, if theexpansion device 55 is an expansion valve 55 as depicted in FIG. 1, thecontroller 100 closes the expansion valve 55 when the unloader flowcontrol device 85 is open, that is when the system is operating in anunloaded cycle, but when the system 10 is operating in a loaded modewith the unloader flow control device is closed and the flow controldevice 75 open, the controller 100 modulates the degree of openness ofthe expansion device 55 to control the flow of refrigerant to theevaporator 50. However, if the expansion device 55 comprises athermostatic expansion valve 57, or a fixed orifice device, incombination with a flow control valve 53 as depicted in FIG. 2, thecontroller closes the flow control valve 53 when the unloader flowcontrol device 85 is open, that is when the system is operating in anunloaded cycle. When the system is operating in a loaded mode, thethermostatic expansion valve 57 controls the flow of refrigerant to theevaporator 50 in a conventional manner in response to the temperature ofthe refrigerant vapor leaving the evaporator 50 as sensed by atemperature bulb 59, typically mounted to refrigerant line 6 downstreamof the outlet of the evaporator 50.

The compression device 20 keeps running during both the loaded cycle andthe unloaded cycle, but during the unloaded cycle, the compressiondevice 20 does not generate a pressure increase from suction todischarge as the refrigerant vapor discharging from the compressiondevice 20 passes back to the suction inlet of the compression device 20through the unload circuit with minimal pressure drop. With the flowcontrol device 75 closed during the unloaded cycle, flow reversal ofrefrigerant in the high-pressure side of the primary refrigerant circuitis prevented. Rather, refrigerant continues to slowly flow from therefrigerant heat rejection heat exchanger 40 through the expansiondevice 55 into the refrigerant heat absorption heat exchanger 50. Toprevent refrigerant flow reversal during operation of the system 10 inthe unloaded cycle, the controller 100 closes not only the flow controldevice 75, but also the flow control device 65, or if the flow controldevice 65 is not present the expansion valve 55 or flow control valve53, and also closes the economizer flow control valve 95, if present.Preventing refrigerant flow reversal during operation promotes a fastand efficient switch from the unloaded cycle of operation to the loadedcycle of operation, because the refrigerant mass in the primaryrefrigerant circuit does not have to redistribute, as would be the caseif the pressures within the refrigerant circuit between thehigh-pressure side (i.e. upstream with respect to refrigerant flow ofthe expansion valve 55) and the low-pressure side (i.e. downstream withrespect to refrigerant flow of the expansion valve 55) had becomeequalized as a result of flow reversal.

The controller 100 also keeps the evaporator fan 54 operating during theunloaded cycle so that air continues to circulate from the cargo storagespace 200 over the heat exchanger coil 52 of the evaporator 50.Therefore, the refrigeration of this air continues even during operationof the refrigerant vapor compression system 10 in the unloaded cycle,albeit at much less refrigeration capacity than the refrigerationcapacity of the system when operating at full load. By repeatedlyalternating operation of the refrigerant vapor compression system 10between a loaded cycle for a first period of time and a unloaded cyclefor a second period of time, the overall refrigeration capacity of thesystem, averaged over time, during the set point control operating modeis a relatively small fraction of the refrigeration capacity of thesystem 10 when operating at full load as during the pull down operatingmode. Likewise, the power consumed by the compression device 20 duringthe unloaded cycle is a relatively small fraction of the power consumedby the compression device 20 when operating at full load as during thepull down operating mode. Keeping the compression device 20 runningduring the unloaded cycle, as opposed to turning the compression device20 off during the unloaded cycle, reduces the number of times thecompressor is started, which extends the expected life of thecompression device by reducing the risk of premature failure and alsoreduces energy consumption due to motor inefficiency during start-up.

Thus, the refrigerant vapor compression system 10 includes a primaryrefrigerant circuit and an unload circuit arranged in parallel withrespect to refrigerant flow. When the system is operating in a loadedcycle, refrigerant flows through the primary circuit from thecompression device 20 through the refrigerant heat rejection heatexchanger 40, thence the expansion device 55 and the refrigerant heatabsorption heat exchanger 50 and before returning to the compressiondevice 20. In the loaded cycle, the unload circuit is closed torefrigerant flow. When the system is operating in an unloaded cycle, theprimary refrigerant circuit is closed to refrigerant flow andrefrigerant flows through the unload circuit from the discharge outletof the compression device 20 to return to the suction inlet of thecompression device, while bypassing the refrigerant heat rejection heatexchanger 40, the expansion device 55 and the refrigerant heatabsorption heat exchanger 50.

In full-load operation, the controller 100 opens both of the flowcontrol devices 65 and 75, and simultaneously closes the unloader flowcontrol device 85, whereby the primary refrigerant circuit is open torefrigerant flow and the unload circuit is closed to refrigerant flow.In part-load operation, the controller 100 modulates the capacity of arefrigerant vapor compression system 10 by first operating thecompression device 20 in a loaded cycle for a first period of time; thenoperating the compression device 20 in an unloaded cycle for a secondperiod of time; and thereafter repeatedly alternating operation of thecompression device 20 between operation in the loaded cycle and theunloaded cycle.

The embodiments depicted in FIGS. 1-3 are meant to be exemplary, notlimiting, of the application of a compression device unloader circuit inparallel with a primary refrigerant circuit of a refrigerant vaporcompression system. It is to be understood that the primary refrigerantcircuit may include other conventional components and associatedrefrigerant circuits. For example, the primary refrigerant circuit couldalso include an interstage cooling circuit in operative association withthe compression device for passing the refrigerant passing from onecompression stage to another compression stage through a refrigerantcooler.

The controller 100 may be an electronic controller, such as amicroprocessor controller, or any other controller of the typeconventionally used in connection with controlling the operation of arefrigerant vapor compression system. For example, in transportrefrigeration applications, the controller 100 could be a MicroLink™series microprocessor controller, such as the ML2 model, the ML2i modelor ML3 model, available from Carrier Corporation, Syracuse, N.Y., USA.It is to be understood, however, that any controller capable ofperforming the functions discussed hereinbefore with respect tocontroller 100 may be used in the refrigerant vapor compression systemof the invention and in carrying out the method of the invention.

The foregoing description is only exemplary of the teachings of theinvention. Those of ordinary skill in the art will recognize thatvarious modifications and variations may be made to the invention asspecifically described herein and equivalents thereof without departingfrom the spirit and scope of the invention as defined by the followingclaims.

1. A refrigerant vapor compression system comprising: a refrigerantcompression device having a refrigerant discharge outlet and arefrigerant suction inlet, a refrigerant heat rejection heat exchangerfor passing refrigerant received from said compression device at a highpressure in heat exchange relationship with a cooling medium, and arefrigerant heat absorption heat exchanger for passing refrigerant at alow pressure refrigerant in heat exchange relationship with a heatingmedium disposed in serial refrigerant flow communication in a primaryrefrigerant circuit; an expansion device disposed in the primaryrefrigerant circuit downstream of said refrigerant heat rejection heatexchanger and upstream of said refrigerant heating heat exchanger; afirst flow control device disposed in the primary refrigerant circuitdownstream with respect to refrigerant flow of the discharge outlet ofsaid compression device and upstream with respect to refrigerant flow ofthe refrigerant heat rejection heat exchanger; an unload circuitoperatively associated with said compression device including an unloadrefrigerant line having an inlet in refrigerant flow communication withthe primary refrigerant circuit at a first location downstream withrespect to refrigerant flow of the discharge outlet of said compressiondevice and upstream with respect to refrigerant flow of said first flowcontrol device and at a second location downstream with respect torefrigerant flow of said refrigerant heat absorption heat exchanger andupstream with respect to refrigerant flow of the suction inlet to saidcompression device, and a unload circuit flow control device disposed insaid unload refrigerant line; and a controller operatively associatedwith said first flow control device and said unload circuit flow controldevice, said controller operative to switch said refrigerant vaporcompression system between a first operating mode wherein saidcompression device operates in a loaded cycle and a second operatingmode wherein said compression device operates in an unloaded cycle.
 2. Arefrigerant vapor compression system as recited in claim 1 wherein thecontroller positions said unload circuit flow control device in a closedposition and positions said first flow control device in an openposition to operate said refrigerant vapor compression system in thefirst operating mode.
 3. A refrigerant vapor compression system asrecited in claim 2 wherein the controller modulates said expansiondevice in the first operating mode.
 4. A refrigerant vapor compressionsystem as recited in claim 1 wherein the controller positions saidunload circuit flow control device in an open position and positionssaid first flow control device in a closed position to operate saidrefrigerant vapor compression system in the second operating mode.
 5. Arefrigerant vapor compression system as recited in claim 4 wherein thecontroller positions said expansion device in a closed position in thesecond operating mode.
 6. A refrigerant vapor compression system asrecited in claim 1 further comprising: a second flow control devicedisposed in the primary refrigerant circuit downstream with respect torefrigerant flow of the refrigerant heat absorption heat exchanger andupstream with respect to refrigerant flow of the suction inlet of saidcompression device.
 7. A refrigerant vapor compression system as recitedin claim 6 wherein the controller positions said unload circuit flowcontrol device in a closed position and positions each of said firstflow control device and said second flow control device in a openposition to operate said refrigerant vapor compression system in thefirst operating mode.
 8. A refrigerant vapor compression system asrecited in claim 7 wherein the controller modulates said expansiondevice in the first operating mode.
 9. A refrigerant vapor compressionsystem as recited in claim 6 wherein the controller positions saidunload circuit flow control device in a open position and positions eachof said first flow control device and said second flow control device ina closed position to operate said refrigerant vapor compression systemin the second operating mode.
 10. A method for modulating the capacityof a refrigerant vapor compression system including a refrigerantcompression device, a refrigerant heat rejection heat exchanger, anexpansion device, and a refrigerant heat absorption heat exchangerdisposed in series flow arrangement in a primary refrigerant circuit,said method comprising the steps of: operating said compression devicein a loaded cycle for a first period of time; operating said compressiondevice in an unloaded cycle for a second period of time; and repeatedlyalternating operation of said compression device between operation inthe loaded cycle and the unloaded cycle.
 11. A method for modulating thecapacity of a refrigerant vapor compression system as recited in claim10 comprising the step of providing a compression device unloadercircuit in parallel refrigerant flow relationship with the primaryrefrigerant circuit, the unloader circuit connecting a refrigerantdischarge outlet of said compression device in direct refrigerant flowcommunication with a refrigerant suction inlet of said compressiondevice.